The present invention relates to environmental control systems. More specifically, the present invention relates to an environmental control system including an ozone-destroying catalytic converter.
A commercial aircraft usually includes an environmental control system for providing a stream of cooled, conditioned air to an aircraft cabin. A typical environmental control system receives compressed air such as bleed air from a compressor stage of an aircraft gas turbine engine, expands the compressed air in a cooling turbine and removes moisture from the compressed air via a water extractor.
Toxic ozone in the compressed air becomes an issue when an aircraft is cruising at altitudes that exceed 20,000 feet. To reduce the ozone to a level within satisfactory limits, the environmental system is provided with an ozone-destroying catalytic converter.
There are a number of desirable characteristics for an ozone-destroying catalytic converter of an aircraft. These characteristics include a) high efficiency of ozone conversion at bleed air operating temperature; b) good poison resistance from humidity, sulfur compounds, oil, dust, and the like, which may be present in the compressed air (for long life and minimum system overhaul and maintenance costs); c) light weight to minimize system parasitic load; d) high structural integrity of catalyst support under extreme heat and/or vibration shock, which may arise during normal flight conditions (also for long life and minimum system overhaul and maintenance costs); and e) high mass transport efficiency with low pressure drop.
An ozone-destroying catalytic converter with a metal core may be washcoated with a slurry of a water-based silica sol and a refractory metal to form an undercoat layer followed by an overcoat layer of alumina oxide. Both layers may then be catalyzed directly by dipping the washcoated core in a catalyst solution having strong acidity. However, the strongly acidity can cause corrosion of the metal core, especially if the core is made of aluminum.
The overcoat layer may be pre-catalyzed and then washcoated onto the core. Using a pre-catalyzed layer can prevent corrosion during the catalyzing process.
Applying the pre-catalyzed overcoat layer can be problematic. For example, it is difficult to control the uniformity of washcoat layer thickness. Unevenness of the layer thickness can cause a pressure drop across the catalytic converter.
Another problem with the pre-catalyzed overcoat layer is poor catalyst utilization efficiency. Washcoating the pre-catalyzed metal oxide can render certain fractions of the catalytic site inaccessible due to the shielding of the binder material. Furthermore, the surface area provided by the undercoat is not utilized to extend the catalyst lifetime. Since poisons in the compressed air can reduce the efficiency of conversion, lifetime and efficiency of the catalytic converter is further reduced because of the poor catalyst utilization efficiency.
Another potential problem with water-based washcoat layers is its limited mechanical durability. A catalytic converter for a commercial aircraft is subjected to high temperatures and large temperature swings (e.g., between 150° F. and 500° F.) during normal flight operation. The catalytic converter is also subjected to high vibrations during normal flight operation. These harsh conditions can cause the washcoat layer to flake off. Consequently, operating life of the catalytic converter is reduced.